The present invention relates to composite doctor blades. More particularly, the present invention relates to composite doctor blades for use in papermaking, for example in calenders during the manufacture of printing paper. The term xe2x80x9ccalenderxe2x80x9d and variations thereof, as used herein, is intended to refer to an apparatus used to calender paper, including stand-alone calendering units such as supercalenders and calendering units within a papermachine such as machine calenders, gloss calenders and soft nip calenders. The present invention further relates to a method of using doctor blades in calenders.
Doctor blades are widely used to remove various materials from the surface of papermachine rolls. By its very nature, the process of removal of contaminants from the roll surface may result in significant wear to the roll surface, the doctor blade itself or both. The components of paper, particularly coating components, tend to be abrasive and tend to cause wear in the surface of the papermachine roll. Conventional doctor blades may be constructed from metal, e.g., steel, stainless steel, nickel or bronze, metal coated with alloy or ceramic material, plastic, or xe2x80x9ccompositexe2x80x9d materials, i.e., fiber-reinforced polymeric materials. FIG. 1 shows a typical papermachine configuration wherein a doctor blade 2 is positioned against a surface 16 of a papermachine roll 12, for example a calender roll. Doctor blades typically have a 45xc2x0 beveled edge 14, as shown in FIG. 1.
Metal blades generally exhibit high stiffness in the machine direction, i.e., the direction perpendicular to the rotational axis of the papermachine roll, and good wear characteristics. The machine direction of the papermaking process is generally known in the art as the direction of the paper web as it passes through the papermachine and is indicated by arrow 18 in FIG. 1. Such blades also tend to be susceptible to corrosion and to cause excessive roll wear.
Plastic blades tend to be used in papermachine locations unsuitable for metal blades. Plastic blades, however, generally have significant drawbacks because they tend to have low stiffness and tend to degrade at the temperatures typically used in the papermaking process.
Composite blades are typically formed from a plurality of fibrous layers impregnated with resin, each fibrous layer generally having a woven structure such that a certain proportion of the fibers lay in the machine direction, while the remaining fibers lay in the cross-machine direction, i.e., the direction parallel to the rotational axis of the papermachine roll. The cross-machine direction is generally known is the art as the direction perpendicular to the path of the paper web and is indicated by arrow 20 in FIG. 1. Although composite blades tend to wear more quickly than metal blades, they also tend to cause less wear on the roll surface. Reduced blade life is typically viewed as a drawback and improved wear resistance of the blade is seen as desirable for many doctoring applications. The wear characteristics of composite doctor blades are generally considered acceptable in many conventional calendering applications because excessive roll wear may deleteriously affect the final properties of the paper.
Composite doctor blades are often used with on-line calenders, which are typically run at relatively high nip pressures and high roll surface temperatures. These operating conditions tend to increase the amount of coating particles and contaminants on the calender roll surface. If the calendering rolls are not doctored on an almost continuous basis, buildup of coating particles and contaminants reach unacceptable levels, directly affecting the final product properties of the paper, such as paper gloss and paper smoothness. Moreover, the abrasiveness of the particles and contaminants tend to degrade the surface of the calender roll, causing a permanent degradation of the roll surface. Degradation in the roll surface tends to cause a deterioration of the roll profile, i.e., the roll surface is uneven which tends to cause inconsistent calendering across the width of the paper web. Thus, the demand for consistent paper quality at a high production rate and with greater efficiency has typically resulted in almost continuous doctoring of the calendering rolls during operation to remove contaminants. As a result, there have been significant efforts to increase the wear resistance and, consequently, the blade life of composite doctor blades.
The operating conditions for on-line calendering have also driven efforts to increase the wear resistance of the calendering rolls. It is becoming more common for such on-line calendering rolls to be coated with a thin layer of thermal spray coating, which typically exhibits resistance to roll surface degradation and, consequently, deterioration of the roll profile. The term xe2x80x9cthermal sprayxe2x80x9d and variations thereof, as used herein, is intended to refer to one of three standard processes: plasma, high velocity oxygen-fuel (HVOF), and detonation gun, whereby a material, typically in powder form, is heated and deposited on a surface. The thermal spray coating tends to be a ceramic or metal matrix coating. The surface of a thermal spray coated roll may also be ground to a very low roughness, a highly desirable property for calendering rolls used in the manufacture of coated printing papers.
Thermal spray coatings tend to resist scratching from doctoring activities when such doctoring activities are performed on an intermittent basis, such as the removal of paper wrapped around a roll after a break in the paper web. A thermal spray coated roll will, however, generally exhibit roll degradation when subjected to almost continuous doctoring. Over time, thermal spray coated rolls tend to exhibit deterioration in the roll profile and surface finish caused by the action of the doctor blade and the contaminants. When the roll profile and surface finish have degraded to an extent such that the quality of the paper is adversely affected, the roll must be removed and reground. Removal for grinding can result in a significant loss to production and increased costs. In addition, the grinding process itself removes a valuable layer of thermal spray coating from the roll. Because the thermal spray coating layer of the roll tends to represent a significant portion of the cost of the roll and a significant monetary investment, minimizing the loss of thermal spray coating is highly desirable.
Efforts to increase the wear resistance of composite doctor blades may result in more rapid deterioration of the surface of the roll. On the other hand, an adequate level of wear resistance is required to minimize disruptions to production caused by the need to change doctor blades. There remains a need for a doctor blade that may be used almost continuously against the surface of a thermal spray coated roll to adequately remove surface contaminants, while exhibiting sufficient wear resistance to be practical in the production setting. There also remains a need for a doctor blade that may be used to maintain a low surface roughness of the roll with minimal deterioration of the thermal spray coating.
The inventor has discovered that a composite doctor blade that includes a plurality of unidirectional fibers, i.e., abrasive fibers aligned in a direction parallel to the long axis of the doctor blade, may be used to remove surface contaminants from the surface of a roll with minimal deterioration of the roll surface. The inventor has found that such a doctor blade may remain in substantially continuous contact with the surface of a roll during operation without significant damage to the surface of the roll.
The composite doctor blade of the invention is suitable for use in the manufacture of paper, particularly for use in calenders. The composite doctor blade of the invention provides the abrasiveness required in paper manufacturing to adequately clean roll surfaces without unacceptable deterioration of the roll surfaces. The doctor blades of the invention exhibit the structural properties required for effectual doctoring, such as stiffness in both axes of the doctor blade. The doctor blade of the invention also tends to wear slowly and uniformly. Embodiments of the doctor blade of the invention may also be used to reduce and maintain a desired level of surface roughness of the roll.
In one aspect, the invention provides a doctor blade including composite material that includes a plurality of unidirectional fibers, impregnated with a resin.
Preferred embodiments may include one or more of the following features. The doctor blade has a laminate structure including multiple layers of composite material. The unidirectional fibers are selected from the group consisting of fiberglass, ceramic, and mixtures thereof. Preferably the fibers are provided as long continuous filaments or multifilament strands. Preferably the fibers are fiberglass. The unidirectional fibers are provided in a unidirectional fabric. At least 60% by weight of the fibers in the unidirectional fabric are unidirectional fibers, preferably 75% by weight, more preferably 90% by weight. The remaining fibers, referred to herein as the secondary fibers, are oriented in a direction other than parallel to the long axis of the doctor blade. The unidirectional fibers have diameters equal to or greater than the diameters of the secondary fibers. Preferably the unidirectional fibers have diameters of about 450 to 1500 xcexcm and the secondary fibers have diameters of about 400 to 700 xcexcm. The unidirectional fabric further includes nonabrasive fibers selected from the group consisting of carbon, i.e., graphite, rayon, aramid, polyester and mixtures thereof. Preferably one or more of the layers of composite material includes carbon fibers aligned in a direction perpendicular to the long axis of the doctor blade. The unidirectional fabric has a weight per unit area of about 230 to 610 g/m2. The impregnating resin is a thermoplastic resin or an epoxy resin, i.e., a resin containing an epoxide, oxirane or ethoxylene group. The resin has a glass transition temperature, Tg, of about 65 to 315xc2x0 C., preferably 85 to 315xc2x0 C. The resin further includes an abrasive additive selected from the group consisting of glass microspheres, glass fibers, crushed glass, synthetic or industrial diamond particles, silica particles, silicon carbide particles, boron particles, zirconium particles, aluminum oxide particles and mixtures thereof.
In another aspect, the invention provides a method of cleaning a roll surface including:
a) positioning a doctor blade having a long axis near the roll surface such that the long axis of the doctor blade is substantially parallel with the rotational axis of the roll, the doctor blade including a plurality of unidirectional fibers, impregnated with resin; and
b) pressing a beveled edge of the doctor blade against the surface of the roll.
In another aspect, the invention features using the above described method, to decrease the roughness of a roll surface.
Preferred methods may include one or more of the following features. The beveled edge of the doctor blade remains in substantially continuous contact with the roll surface during operation. The positioning step includes the formation of an angle of about 25 to 30xc2x0 between the beveled edge of the doctor blade and the roll surface, as measured from a tangent to the roll where the beveled edge touches the roll surface. The pressing step is performed at a pressure of about 85 to 700 N/m, preferably about 175 to 440 N/m. The surface roughness of the roll is reduced to about 0.025 to 0.20 xcexcm Ra, preferably about 0.050 to 0.13 xcexcm Ra . The surface roughness of the roll is maintained during the effective life of a blade at a level of about 0.025 to 0.20 xcexcm Ra, preferably about 0.050 to 0.13 xcexcm Ra.
In another aspect, the invention provides a method of making a composite doctor blade including the step of impregnating a composite material comprising unidirectional fibers.
Preferred methods may include one or more of the following features. The method includes a layering step wherein multiple layers of composite material are superimposed on top of one another to form a laminate structure. The method includes a curing step wherein the resin is subjected to an elevated temperature and pressure. The method includes a cutting step wherein the cured laminate structure is cut into two or more doctor blades, each blade having a long axis.